In modern society, X-ray has been widely used, for example in CT scanning apparatus, for scanning and imaging many objects. Generally, conventional X-ray scanning and imaging technique uses X-ray attenuation through materials to nondestructively detect interiors of objects. The more different in density of internal components of the object are, the greater will be the effect of the conventional X-ray imaging technique. Substances consisting of light elements have weak absorbing abilities for X-ray, thus conventional X-ray imaging technique can hardly identify their internal structures. In this case, other auxiliary means, such as injecting contrast agent into biological tissues, do not help to obtain clear images, without which a lot of inconveniences may be caused. In the 1990s, there appeared an X-ray phase-contrast imaging technique which uses information concerning the phase shift of an X-ray beam to observe changes in density of electrons in an object, thereby determining the inner structure of the object. Generally, the early phase-contrast imaging methods use interference or diffraction of coherent or partially coherent X-rays to improve the low-contrast resolution of the radiation image. On such a basis, in the patent applications CN101532969A entitled “System and method for X-ray grating-based phase-contrast imaging” (Patent Reference 1) and CN101726503A entitled “X-ray phase contrast tomography imaging” (Patent Reference 2), wherein all the contents of said patent applications are incorporated into the present application by reference, HUANG Zhifeng et al. put forward a novel technical concept for incoherent grating-based phase-contrast imaging. Specifically, the said references use two absorption gratings which can translate relative to each other for several steps within a range of one grating period, and a detecting device acquires one image for each translation step; after the image acquisition process for one grating period has been finished, for each pixel, the sample intensity curve and the background intensity curve are compared such that the information concerning the refraction image of the object to be detected can be calculated. This approach has a good phase-contrast imaging effect. Said approach can be performed with multicolored and incoherent X-ray sources and thus can be embodied as simple and easy devices.
Furthermore, during the progress of the X-ray imaging technology, there also appeared a dark-field imaging technique. Said dark-field imaging technique uses indirect light such as scattered light, diffracted light, refracted light, fluorescent light and the like to illuminate objects, and then form images of the internal structures of the objects by means of the difference in their capabilities of scattering X-rays. Generally, the dark-field imaging with hard X-rays is difficult to well perform, since the special optical properties of hard X-rays make it is difficult to manufacture optical components required for dark-field imaging with hard X-rays. However, the dark-field imaging with hard X-rays has a better capability to identify and detect the internal microstructures of objects than the bright-field imaging and the phase-contrast imaging. Since the scattering of the hard X-rays is at a micrometer level or a nanometer level, the dark-field imaging with hard X-rays can be used to identify the internal ultrafine structures of objects, which, in contrast, cannot be determined by the bright-field imaging and phase-contrast imaging with hard X-rays. In 2009, in the patent application CN101943668A entitled “X-ray dark-field imaging system and method” (Patent Reference 3), wherein all the contents of said patent application are incorporated into the present application by reference, HUANG Zhifeng et al. put forward a technical solution that performs dark-field imaging of objects by using X-rays. Specifically, the technical solution of the said reference comprises: emitting X-rays to an object to be detected; enabling one of two absorption gratings to perform stepping within at least one period; for each step, the detecting device receiving and converting X-rays into an electrical signal; after stepping over at least one period, representing the X-ray intensity at each pixel of the detecting device as an intensity curve; comparing, at each pixel of the detecting device, the intensity curve with the object to be detected and the intensity curve without the object, and calculating the second moment of the scattering angle distribution at each pixel; taking images of the object from different angles, and then obtaining a scattering information image of the object according to a CT reconstruction algorithm.
The above-mentioned grating-based imaging techniques require the stepping process to obtain the intensity curve at each detection unit (pixel) of the detecting device. The basic principle of the stepping technique is: after a source grating is fixed adjacent to an X-ray source, in the technique based on a Talbot-Lau interference method, a phase grating or resolution grating is relatively translated for several steps within a range of one grating period; while in the technique based on a classic optical method, two absorption gratings are translated relative to each other for several steps in a range of one grating period. The detecting device acquires one image for each translation step. After finishing the image acquisition process for one grating period, for each pixel, the sample intensity curve and the background intensity curve are compared such that the refraction image information, attenuation image information and dark-field image information can be calculated. Generally, conventional stepping technique comprises translating the phase gratings, the resolution gratings or the absorption gratings. In 2010, in the patent application CN102221565A entitled “X-ray source grating-stepping imaging system and imaging method” (Patent Reference 4), wherein all the contents of said patent application are incorporated into the present application by reference, HUANG Zhifeng et al. put forward a grating stepping method for an X-ray source. Specifically, since the source grating has a period of dozens of micrometers, the above approach requires a substantially lower stepping accuracy as compared to the conventional stepping methods.
Nevertheless, the stepping technique adversely affects application of grating-based imaging technique when they are used in combination. A stepping process by means of mechanical systems will be time-consuming and thus increase scanning time. Moreover, even for a stepping action of about a few tens of micrometers, high equipment accuracy, good shock absorption and an appropriate environment temperature will be required. Both of the above factors impede the application of those new grating-based imaging techniques.